PI: Joanna Barstow

Uncovering signs of an Io-like exoplanet

With the discovery of almost 6000 exoplanets to date, an emerging class of particular interest is the subset of planets that sit between the Earth and Neptune in mass. Some of these objects, for which no example exists in the solar system, are likely to be miniature Neptunes, with hydrogen-dominated atmospheres; others may boast envelopes with a large fraction of water; and some will be rocky worlds with secondary atmospheres that have outgassed from the planet’s interior. One such planet is L98-59 d, a planet with a radius 1.5 times that of Earth, orbiting a nearby red dwarf star. Too warm to be a temperate, possibly habitable world, it was initially identified as a possible ‘exo-Venus’ candidate due to its similar temperature. 

We obtained a transmission spectrum of this planet’s atmosphere via the JWST NIRSpec Guaranteed Time Observation programme 1224. Transmission spectra of exoplanets can be obtained when the planet passes in front of (transits) its parent star from our perspective; tiny wavelength-dependent fluctuations in the amount of light blocked by the planet during transit encode fingerprints of absorbing gases in the planet’s atmosphere. To identify, and measure the abundances of, these gases, we need to generate several tens of thousands to millions of individual computational models of the planet’s atmosphere, which we compare to the observations using a Bayesian framework. Project co-I Agnibha Banerjee performed this spectral retrieval analysis for L98-59 d using resources on DiRAC DIaL3. 

Far from identifying an exo-Venus, the atmospheric composition we measured for L98-59 d resembles an entirely different object in our solar system – Jupiter’s moon, Io. During its orbit around Jupiter, Io experiences varying tidal forces, which have the effect of squeezing the planet and depositing heat in its interior. This heat deposition drives high levels of volcanic activity, with the result that Io’s atmosphere contains large amounts of sulphur dioxide, a gas produced by volcanoes. We were surprised to also identify large amounts of this gas, alongside hydrogen sulphide, in the atmosphere of L98-59 d! However, it actually makes a lot of sense. L98-59 d orbits its parent star with a period of just 7.5 days, and its orbit is not perfectly circular, meaning that like Io it is subject to varying tidal forces. Our detections are still tentative at this stage, but if subsequent observations confirm our findings, L98-59 d is likely to be an ‘exo-Io’. 

This figure shows the JWST data (black points with error bars) and our best fitting model (red line) with the contributions from different gases shown (shading).